In a conventional discharge surface treatment, an attention is mainly paid to wear resistance at an ordinary temperature, and a coating consisting of a hard material such as titanium carbide (TiC) is formed (see, for example, Patent Document 1).
Patent Document 1: International Publication No 99/85744 pamphlet
However, a demand for not only forming the hard ceramics coating intended to ensure a high wear resistance at the ordinary temperature but also for forming a coating as thick as 100 μm or more is getting stronger. Functions required for the thick coating include wear resistance and lubricity under high temperature environment. The thick coating having these functions is formed for a component used under a high temperature environment.
To form such a thick coating, an electrode formed by compressing powder mainly consisting of metal powder and, if necessary, subjecting the compressed powder to a heat treatment is used. The electrode differs from the electrode mainly consisting of ceramics and used to form the hard ceramic film.
To form a thick coating by a discharge surface treatment, it is necessary for the electrode to have predetermined properties such as a somewhat low hardness. This is because it is necessary to supply a large amount of the electrode material to the workpiece by discharge pulses.
Although a coating is normally stably formed by the discharge surface treatment, the state of forming a coating could suddenly turn unstable, and once this happens, it is impossible to restore a stable state. The reasons are considered as follows. The sudden occurrence of the unstable state results from concentration of discharge. Once the state turns unstable, a part of the electrode on which the discharge concentrates is widely molten and resolidified. If the part of the electrode is molten, an electrode form of the part is deformed and a discharge is apt to occur to the part.
A discharge surface treatment method according to one aspect of the present invention is for generating a pulse-like discharge between an electrode and a workpiece, using a green compact formed by compressing any one of a metal powder, a metal compound powder, and a ceramics powder as the electrode, and forming a coating consisting either one of a material for the electrode and a material obtained from a reaction of the material for the electrode by a discharge energy of the pulse-like discharge on a surface of the workpiece. The discharge surface treatment method includes detecting a voltage between the electrode and the workpiece during a discharge; and determining that a state of the discharge surface treatment is abnormal if the voltage is lower than a possible predetermined value of a sum of an arc voltage and a voltage drop of the electrode during a discharge in which melting of a local part of the electrode due to concentration of the discharge or resolidification of the local part subsequent to the melting does not occur.
According to the present invention, a voltage between the electrode and the workpiece during the discharge is detected, and it is determined that a discharge surface treatment state is abnormal if it is detected that the voltage is lower than a possible predetermined value of a sum of an arc voltage and a voltage drop of the electrode during the discharge, during which melting of a local part of the electrode due to concentration of the discharge or resolidification of the local part subsequent to the melting does not occur. Thus, an unstable phenomenon is accurately detected during the discharge surface treatment. It is therefore possible to take appropriate measures before the coating film formation state and the electrode state are worsened due to the unstable phenomenon in the discharge surface treatment. Namely, by discriminating the stability of the discharge surface treatment, the coating film and the electrode can be prevented from being damaged.
The present invention has been achieved to solve the conventional problems. It is an object of the present invention to provide a discharge surface treatment method and a discharge surface treatment apparatus that can accurately detect an unstable phenomenon in forming a coating, and that can take appropriate measures before a coating state and an electrode state are worsened due to the unstable phenomenon.